Variable speed constant frequency ("VSCF") systems and other high power airborne applications require the use of high voltage, high frequency, high capacitance capacitors in small packages to filter current produced by these systems. Ceramic capacitors offer several advantages over other types when used in conjunction with power electronics. In fact, large multilayered ceramic capacitor assemblies can be used to replace several smaller capacitors wired in parallel.
These ceramic capacitor assemblies are approximately rectangular in shape, having a width and heighth typically in excess of an inch and a depth of somewhat less than an inch. These capacitor assemblies are provided with electrode coatings on opposite faces where electrical contact may be made to capacitor plates within the capacitor. Apart from electrode ends, the body of the typical ceramic capacitor assembly is coated with a dielectric material.
Mounting of these large ceramic capacitor assemblies (hereinafter referred to as "capacitors") to a printed circuit board, chassis wall or other planar surface has proven to be a difficult task in the past. Because of their size, the capacitors must be mechanically fixed to the planar surface. Furthermore, due to the high quantity of power passing through the capacitors, significant electrical contact must be made to the capacitor electrodes.
Mounting was done typically in two steps. First, the body of the capacitor was usually mechanically fixed to the planar surface by cement or by pooling solder on the planar surface and pressing the dielectric body of the capacitor into the solder to enable the solder to hold the capacitor. Second, electrical contact was accomplished by bringing electrical leads from the planar surface to the electrodes on the capacitor. Accordingly, mounting of large ceramic capacitors was by trial and error and had to be repeated individually for each capacitor to be mounted.
Apart from an unsightly mess on the printed circuit board and a constant problem arising from mounting solder creating undesired electrical paths on the board, a further problem encountered with the prior mounting methods was that significant thermal expansion of the capacitor would occur during operation. Accordingly, extreme stress would be imparted to the cement or solder holding the body of the capacitor to the planar surface. Sometimes the cement or solder would loosen, allowing the capacitor to separate from the planar surface. At other times, the cement or solder would not allow the capacitor to expand, causing catastrophic failure.
Further, the old mounting method failed to provide a means by which the capacitors could be cooled during operation, thereby relieving stress on the capacitors and lengthening their lives.
Finally, the old mounting method failed to provide a simple means by which capacitors could be replaced following failure. Under the old method, the solder or cement holding the capacitor in place would have to be melted or dissolved and the new capacitor would then have to be installed in the same manner as the replaced capacitor.
For years, low power applications have demanded mounting of capacitors to printed circuit boards. However, low power applications do not have to address the mechanical mounting, power handling, thermal expansion or replacement problems encountered in high power applications.
Following are patents which address capacitor mounting for low power applications.
U.S. Pat. No. 4,401,843 ("'843"), which issued on Aug. 30, 1983 to Harper et al., is directed to various structures for, and methods of constructing, miniaturized high capacitance bus bars. The bus bars incorporate discrete high capacitive elements between a pair of bus bar conductors. Alternative arrangements are presented wherein the capacitive elements are maintained in electrical contact with the conductors while avoiding the development of short circuits between the two bus bar conductors. Harper et al. 'fails to teach a structure for mounting large ceramic capacitors to a printed circuit board in which the capacitors may be easily removed in case of failure.
The subject invention, in contrast, is specifically directed to provision of a high power, high temperature disassemblable capacitor mount which permits high power ceramic capacitors to be mounted to a printed circuit board. As such, the subject invention must address problems associated with mechanical mounting of large capacitors, thermal expansion and cooling. Specifically, the subject invention provides for the capacitors to be removable from the bus bars to thereby allow the capacitors to be replaced in case of failure. Compressible, electrically conductive contactors are provided to permit the capacitors to thermally expand during operation, while maintaining electrical contact with the electrodes on the capacitors. Harper et al. '843 fails to teach the structure and the purposes of the subject invention.
U.S. Pat. No. 4,451,694 ("'694"), which issued on May 29, 1984 to Harper et al., is directed to various structures for, and methods of constructing, miniaturized high capacitance bus bars. The bus bars incorporate discrete high capacitive elements between a pair of bus bar conductors. In a preferred embodiment presented in this continuation-in-part of Harper et al. '843, a zebra film comprised of an elastomeric material having alternating strips of conducting and non-conducting material is positioned between the capacitive elements and the bus bar conductors. This zebra structure establishes electrical contact between the capacitive elements while avoiding the development of short circuits between the two bus bar conductors. This zebra structure may also have a mechanical force or means applied whereby the applicable conductive surfaces are urged into electrical contact with the elastomeric strip material.
The subject invention is directed to a disassemblable capacitor mount for high power, thermally expansive capacitors. Accordingly, the subject invention must provide a capacitor mount which is able to take significant thermal stress. Harper et al. '694, does not teach a structure which is able to take significant thermal stress. Further, Harper et al. '694, does not show a structure which allows cooling to take place between spaced-apart capacitors. Finally, the subject invention provides a high power electrical contact for the capacitors. Harper et al., '694, specifically directed to a low power capacitor mounting arrangement.
U.S. Pat. No. 4,517,406, which issued on May 14, 1985 to Erdle, is directed to one or more multi-layer ceramic capacitors positioned in a laminated bus bar so that a pair of opposed, external electrodes on each capacitor is positioned in substantially coplanar arrangement with the confronting plane surfaces of adjacent conductor strips in the bar. The two external electrodes of each capacitor are connected each to a different one of two spaced metal termination plates, which are mounted on each capacitor to interconnect its alternate and intervening electrodes, respectively. The two termination plates of each capacitor register with the spaced recesses or openings formed in each of the adjacent conductor strips to accommodate any projections on the termination plates and thereby permit coplanar engagement of the external electrodes with the conductor strips.
Again, the subject invention is directed to removable mounting and fixturing of high power ceramic capacitors. Erdle is not directed to and, accordingly, Erdle fails to teach the structure of the subject invention. Erdle specifically fails to teach a stress alleviating structure for mounting capacitors which allows capacitors to readily removed and replaced.
U.S. Pat. No. 4,599,486, which issued on July 8, 1986 to Herrandez, is directed to a miniaturized surface mountable bus bar wherein a sheet of insulating material is laminated between a pair of bus conductors and windows are provided in the laminated structure. A plurality of multi layer ceramic capacitor elements is inserted in the windows and alternate conductive side plates of the capacitors are electrically connected to the two bus conductors. The capacitors are comprised of alternating layers of conductive material and dielectric material and dielectric material having opposed terminating side conductors which are oriented parallel to the sheet of insulating material and the pair of bus conductors after assembly thereof.
The subject invention has structure which is substantially different from the structure taught in Herrandez. The structure in Herrandez is not directed to removable securing of large, high power ceramic capacitors to a printed circuit board and, therefore, fails to allow for thermal expansion and cooling of the capacitors and for easy removal and replacement of faulty capacitors.